The present invention relates to color optical scanners in general and more specifically to variable resolution, single pass color optical scanners.
Color optical scanners are similar to black and white optical scanners in that data signals representative of the object or document being scanned are produced by projecting an image of the document onto an optical photosensor array. The data signals may then be digitized and stored for later use. For example, the data signals may be used by a personal computer to produce an image of the scanned object on a suitable display device.
Most optical scanners use illumination and optical systems to illuminate the object and focus a small area of the illuminated object, usually referred to as a "scan line," onto the optical photosensor array. The entire object is then scanned by sweeping the illuminated scan line across the entire object, either by moving the object with respect to the illumination and optical assemblies or by moving the illumination and optical assemblies relative to the object.
A typical scanner optical system will include a lens assembly to focus the image of the illuminated scan line onto the surface of the optical photosensor array. Depending on the particular design, the scanner optical system may also include a plurality of mirrors to "fold" the path of the light beam, thus allowing the optical system to be conveniently mounted within a relatively small enclosure. In order to allow a smaller photosensor array to be used, most optical systems also reduce the size of the image of the scan line that is focused onto the surface of the photosensor. For example, many optical systems have a lens reduction ratio of about 8:1, which reduces the size of the image of the scan line by a factor of about 8.
While various types of photosensor devices may be used in optical scanners, the most common sensor is the charge coupled device or CCD. As is well-known, a CCD may comprise a large number of .individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the size of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. In a typical scanner application, the charge built up in each of the CCD cells or pixels is measured and then discharged at regular intervals known as sampling intervals, which may be about 5 milliseconds for a typical scanner.
In most optical scanner applications, each of the individual pixels in the CCD are arranged end-to-end, thus forming a linear array. Each pixel in the CCD array thus corresponds to a related pixel portion of the illuminated scan line. The individual pixels in the linear photosensor array are generally aligned in the "cross" direction, i.e., perpendicular to the direction of movement of the illuminated scan line across the object (also known as the "scan direction"). Each pixel of the linear photosensor array thus has a length measured in the cross direction and a width measured in the scan direction. In most CCD arrays the length and width of the pixels are equal, typically being about 8 microns or so in each dimension.
As mentioned above, each pixel in the CCD array corresponds to a related pixel portion of the illuminated scan line on the object. To avoid confusion, the corresponding pixel portion on the illuminated scan line will be referred to herein as a "native object pixel" or simply "native pixel." A native object pixel has dimensions equal to the dimensions of the corresponding pixel on the linear photosensor array multiplied by the lens reduction ratio of the optical system. For example, in a scanner having a CCD pixel size of 8 microns by 8 microns and a lens reduction ratio of 8:1, the size of the native object pixels will be about 64 microns by 64 microns. Also, the linear array of native object pixels that corresponds to the linear array of CCD pixels will be referred to herein as a "native scan line."
Scanners are typically operated at a scan line sweep rate such that one native object pixel width (i.e., a native scan line) is traversed during each CCD sampling interval. However it has been discovered, as disclosed in Meyer et al., U.S. Pat. No. 5,047,871, which is hereby specifically incorporated by reference for all that it discloses, that the resolution of a display image produced with data generated by some scanners may be varied by changing the scan line sweep rate of the scanner. For example, by increasing the scan line sweep rate from one native scan line per sampling interval to two native scan lines per sampling interval, each CCD is exposed to two native scan line widths during a single sampling interval. As a result, the size (as measured along the scan direction) of the image resulting from the faster scan speed is one-half the size of a display image produced at the slower scan speed. Put in other words, the increased scan speed results in an effective increase in the width of object pixels, which also corresponds to a decrease in resolution along the scan direction. The ability to vary the resolution along the scan direction and/or "scale" the image produced by a display device by controlling scanner sweep speed is a significant feature which is offered on some newer scanners.
Color optical scanners differ from the black and white scanners described above in that multiple color component images of an object must be collected to produce a color display image of the object. For example, data representative of red, green, and blue color components of the image of the scan line may be produced, correlated, and stored by the scanner apparatus.
Many different techniques have been developed for collecting data representative of multiple color component images of the object being scanned. One technique projects the image of the illuminated scan line onto a single linear sensor array in much the same way as for black and white scanners. However, in order to collect the multiple color component images of the illuminated scan line, a different color light source is used to illuminate the scan line on each of many scanning passes. For example, the object first may be scanned using only red light, then only green light, and finally only blue light. In a variation of this technique, three scanning passes are made using a white light source, but the light from the illuminated scan line is filtered by a different color filter during each of the three passes before being focused onto the optical photosensor array.
Another technique, described in U.S. Pat. No. 4,709,144 issued to Vincent and U.S. Pat. No. 4,926,041, issued to Boyd, et al., both of which are hereby specifically incorporated by reference for all that is disclosed therein, is to split the illuminated (i.e., polychromatic) scan line into multiple color component beams, each of which are then focused onto multiple linear photosensor arrays. For example, the illuminated scan line may be split into red, green, and blue color component portions which are then simultaneously projected onto three (3) separate linear photosensor arrays. This technique allows the component color image data generated from any particular scan line to be generated simultaneously, thus allowing easier correlation of the image data for each separate color component.
Yet another technique for generating multiple color component images from a polychromatic or white light source is described by Takeuchi, R. et al. (1986), in "Color Image Scanner with an RGB Linear Image Sensor," SPSE Conference, The Third International Congress On Advances in Non-Impact Printing Technologies, PP339-346, August 1986, which is hereby specifically incorporated by reference for all that it discloses. Essentially, Takeuchi simultaneously projects light from different scan line regions of the object onto separate linear photosensor arrays, each of which is covered with a different color filter. With this technique it is necessary to first correlate the data representative of different scan line component images since the different component color images of any scan line region of the document are generated at different times.
Regardless of the particular technique used to collect data representative of multiple component color images, there remains the problem of correlating the data for the various color component images so that they correspond to the same illuminated scan line on the object. One solution to the problem is to allow the scanner to scan only at those scan rates, known as "native scan rates," that allow the color image data to be easily correlated by the image processing system. Unfortunately, there are a limited number of native scan rates that will allow for such simple color data correlation. Since the resolution of the scanner is related to the scanning rate, a scanner limited to scanning at a few native scan rates will be limited to a corresponding few scan resolutions, known as "native scan resolutions," thus significantly reducing the utility of the scanner.
One method that will allow for scanning at resolutions other than native scan resolutions is to select the next higher native scan resolution that will allow for simple color data correlation, and then drop data from selected pixels, or otherwise process the data to achieve the desired resolution. Unfortunately, this method can result in decreased image quality unless relatively intensive data processing operations are performed to enhance the image data.
Still another method that will allow for scanning at various predetermined resolutions is disclosed in U.S. Pat. No. 5,336,878 issued to Boyd, et al., which is specifically incorporated herein by reference for all that it discloses. While the method and apparatus disclosed in Boyd has many advantages, the color components of the resulting image can still be mis-aligned by up to 1/2 an effective scan line width.
Consequently, there remains a need for a variable resolution color scanner that can properly correlate the color image data over a wide range of non-native scan resolutions, thus scan rates, but without any remaining color mis-alignment.